The present invention relates to an alignment method used when patterns on a mask are transferred on a substrate such as a wafer in the lithographic process for fabricating, for example, semiconductor devices, charge coupled devices (CCDs), liquid crystal display devices, or thin-film magnetic heads and also relates to an exposure apparatus that uses the alignment method. The invention is particularly adapted to be applied to and suitable for the case where patterns on a mask are aligned with shot regions on a wafer, based on array coordinates predicted by using a statistical method.
A semiconductor device, for example, is formed by overlaying or superimposing a multilayer circuit pattern on a wafer. Therefore, when the second circuit pattern and the circuit patterns thereafter are exposed and transferred on the wafer, the alignment between patterns on the mask (or reticle) which are going to be exposed and each shot region or area on which a circuit pattern has already been defined, that is, wafer alignment has to be performed with a high degree of accuracy.
As an alignment sensor for wafer alignment, there is an alignment sensor of a laser step alignment (LSA) type in which a laser beam is irradiated on an alignment mark in the form of a dotted line (a row of a plurality of dots) on a wafer and the position of the mark is detected by using the beam diffracted or scattered by the mark, or a field image alignment (FIA) type in which the image data of an alignment mark, illuminated and imaged with light having a wide waveband width and emitted from a light source consisting of a halogen lamp, is processed and measured, or a laser interferometric alignment (LIA) type in which laser beams having identical frequencies or slightly different frequencies are irradiated on alignment marks in the form of diffraction grating on a wafer in two directions, two diffracted beams generated interfere with each other, and the position of the alignment mark is measured from the phase difference.
FIG. 1 shows the schematic structure of a projection exposure apparatus equipped with a conventional alignment sensor of the LSA type. In the figure the pattern of a reticle R is transferred on each shot region or area on a wafer W through an projection optical system PL. The wafer W is placed on a wafer stage (not shown) through a wafer holder (not shown). It is assumed that a Z-axis is taken parallel to an optical axis of the projection optical system PL, and a rectangular or orthogonal coordinate system on a plane perpendicular to the Z-axis is represented by an X-axis and a Y-axis. Alignment illumination light beams LX and LY from the LSA and through-the-lens (TTL) types of X-axis and Y-axis alignment sensors 11X and 11Y are irradiated as slit beams SBa and SBb on the edges of an exposure field IAR of the projection optical system PL through the projection optical system PL from the lower portion of the reticle R. With respect to an image formation plane of the projection optical system PL, a best focal plane of the alignment sensors 11X and 11Y is set at the same focal position.
In addition, a focal position detecting system of oblique incidence type which comprises a light emitting system LES and a light receiving system LRS is disposed. Measuring beams LE from the light emitting system LES are irradiated, in an oblique direction, as spot beams on a plurality of measurement points on the substantially central portion of the exposure field IAR, and the reflected beams from the plurality of measurement points are received by means of the light receiving system LRS. The plurality of spot beams are imaged again in the light receiving system LRS, and a plurality of focal signals corresponding to the lateral offset quantities of the images formed again are output. If the position (Z-position) of the wafer W in the direction of the optical axis of the projection optical system PL is changed, the lateral offset quantities in images which are again formed will change and therefore the Z-positions of the wafer W at the plurality of measurement points will be measured from the plurality of focal signals.
FIG. 2 shows the concrete dispositions on the exposure field IAR of the spot beams by means of the measuring beams LE emitted from the focal position detecting systems LES, LRS. As shown in FIG. 2, spot beams 5a through 5e are irradiated on five measurement points equally spaced on approximately the diagonal line of the exposure field IAR of the projection optical system PL of FIG. 1. The slit beams SBa and SBb created by the alignment illumination light beams LX and LY emitted from the alignment sensors 11X and 11Y of FIG. 1, are irradiated substantially on the centers of the edges in -Y and +X directions of the exposure field IAR.
An X-axis wafer mark WMa of LSA type and a Y-axis wafer mark WMb of LSA type are formed on the edge in the -Y direction (in FIG. 2 the lower edge) and edge in the +X direction (in FIG. 2 the right edge) of a shot region or area 8 enclosed by dotted lines on the wafer W, respectively. At the time of alignment, the wafer W is moved through a wafer stage (not shown) so that the wafer mark WMa crosses the slit beam SBa in the X direction and then it is moved through the wafer stage so that the wafer mark WMb crosses the slit beam SBb in the Y direction. With the movement of the wafer W, the X and Y coordinates of the wafer marks WMa and WMb are detected. Thereafter, at time of exposure, the center of the shot area 8 on the wafer W is aligned with the center (center of exposure) of the exposure field IAR and, based on the focal signals from the focal position detecting systems LES and LRS, the average Z-position of the shot area 8 is aligned with the image formation plane (previously obtained) of the projection optical system PL. In this way, the pattern images on the reticle R are exposed and transferred onto the shot area 8.
As shown in FIG. 2, the conventional focal position detecting system of oblique incidence type is contemplated to detect the average Z-position of the shot region at the time of exposure. For this reason, only five spot beams 5a through 5e are irradiated on the diagonal line on the exposure field IAR.
In the aforementioned prior art when the alignment of the shot area 8 on the wafer is performed, none of the five spot beams 5a through 5e are irradiated near the wafer marks WMa and WMb in the state where the wafer marks WMa and WMb have crossed the alignment slit beams SBa and SBb. Therefore, the Z-positions on the wafer marks WMa and WMb have not been detected at the time of alignment. For example, in the case where foreign substances such as resist dust particles exist between the wafer and the wafer holder, the Z-positions of the wafer marks WMa and WMb do not always match the best focal positions of the alignment sensors 11X and 11Y and therefore there is the fear that a focus offset will occur at the time of alignment. If the telecentric properties of the alignment sensors 11X and 11Y are lost when an offset such as this occurs, the disadvantage that errors (jump of data measured) will be contained in measured values will exist.
On the other hand, an autofocus device disposed in the conventional exposure apparatus is structured to perform focus measurement (a measurement of a location of a projection optical system on a wafer surface in a direction along an optical axis) within a range on the order of an exposure field size and adjust the position or level of the wafer surface in the direction along the optical axis to an average focal position within the range. Flatness of substrates such as wafers to be exposed can be measured using focus measurement results obtained with the autofocus device.
A degree of roughness of a wafer surface is changed by exposure and subsequent processes, and a step between an alignment mark (wafer mark) and a surrounding base may differ depending on a layer on the wafer. To perform accurate alignment by detecting a position or level of the alignment mark accurately, it is therefore necessary to align a detection region of the alignment sensor with a focal plane of an objective lens thereof (bring the objective lens into focus).
However, the conventional autofocus device which performs focus measurement within the range of the exposure field size and focuses to the average focal position within the exposure field size can hardly measure a true defocus quantity within a minute region on the order of a detection region of the alignment sensor and perform an accurate focus adjustment within this minute region.
Further, an exposure apparatus of a step-and-repeat type (or a step-and-scan type) adopts an enhanced global alignment method (hereinafter referred to as an EGA method), which in order to perform alignment between a projected image of a pattern on a reticle and a chip pattern on the shot region or area of a wafer, measures positions of alignment marks attached to a plurality of specific shots (sample shots) on a wafer such as those disclosed, for example, in Japanese Patent Laid-Open Publication No. Sho 61-44429 using an alignment sensor and determines array coordinates of all the shots on the wafer by statistical processing using the method of least squares on the basis of actually measured values and a design value of each shot array.
However, the conventional autofocus device can hardly measure a true defocus quantity within the minute region on the order of the detection region of the alignment sensor and adjust a focal position accurately within this minute region. Therefore, there may be caused an undesirable situation that, shot areas within which foreign matter such as dust adheres to surfaces thereof and shot areas having remarkable steps formed between marks (in case of multiple marks) within the alignment marks due to roughness on a surface of a wafer are selected as the sample shots. In such a case, the conventional autofocus device is inconvenient in that it involves errors in positional measurements of the alignment marks, thereby resulting in errors in shot array coordinates determined by the EGA method.